serialization: scene save/load (Unity-style YAML + UE-style GUID registry)

Engine-layer SceneSerializer (yaml-cpp submodule): entities/components
to versioned YAML, atomic clear-and-rebuild on load with per-entity
fault tolerance, missing-field defaults, unknown-component skip and
UUID dedup. Asset references are {Guid, Path} pairs resolved through
the new AssetManager GUID registry (assets/asset_registry.yaml) with
path fallback; procedural:// pseudo-paths rebuild from parameters.
Editor File menu gets Save/Load Scene modals; app re-resolves stale
entity handles via onSceneLoaded callback.

App/main.cpp

@@ -201,6 +201,10 @@ namespace
 
         void OnUpdate(const FrameTime& frameTime) override
         {
+            // 加载过无相机实体的场景（或场景被清空）：相机句柄悬空，
+            // 直接跳过本帧游戏逻辑（渲染仍以编辑器视角进行）
+            if (!m_Camera.IsAlive()) return;
+
             // 相机（RMB 飞行 / MMB 焦点环绕 / 滚轮推拉；位姿读写实体
             // Transform）。编辑器相机走真实轨：暂停（timeScale=0）时仍
             // 可飞行观察。编辑器模式移动键需 RMB 按住（UE 语义：飞行时
@@ -268,6 +272,23 @@ namespace
             ctx.app = this;
             ctx.getEnvIndex = [this] { return m_EnvIndex; };
             ctx.setEnvIndex = [this](int index) { m_EnvIndex = index; };
+            // 场景加载成功 → 沙盒侧悬空句柄重解析（registry 清场重建后
+            // m_Camera 等成员句柄全部失效，不重解析则下一帧必崩）
+            ctx.onSceneLoaded = [this] {
+                Scene& scene = GetScene();
+                m_Spinners.clear();
+                m_Gun = {};
+                m_Ground = {};
+                m_KeyLight = {};
+                entt::entity camEnt = entt::null;
+                scene.Each<CameraComponent>([&](entt::entity e,
+                                                 CameraComponent& cc) {
+                    if (cc.primary) camEnt = e;
+                });
+                m_Camera = scene.GetEntityHandle(camEnt);   // 无效 → 空句柄
+                ENGINE_INFO("SandboxApp: scene state re-resolved (camera {})",
+                            m_Camera.IsAlive() ? "found" : "missing");
+            };
             // 继承原沙盒默认：初始暂停（StatsPanel 的 Pause Simulation 开关恢复）
             Time::SetTimeScale(0.0f);
             m_Editor->Init(ctx);